Downgrade C++ standard requirement to C++17

Summary: related to T13242

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D26860

example/receiver.cpp

@@ -10,6 +10,8 @@
 
 #include <async_mqtt5.hpp>
 
+#ifdef BOOST_ASIO_HAS_CO_AWAIT
+
 namespace asio = boost::asio;
 
 asio::awaitable<void> client_receiver(asio::io_context& ioc) {
@@ -78,3 +80,5 @@ int main() {
 }
 
 //]
+
+#endif